DC-DC converters are typically used to convert an uncontrolled input DC voltage to a controlled output DC voltage and supply the controlled output DC voltage to a load. The DC-DC converters are also providing galvanic insulation between the input side and the output side.
WO2011/074977 describes a resonant DC-DC converter with a resonant tank having a transformer, where a switching device is provided for reconfiguring the secondary windings of the transformer between a star configuration and a delta configuration.
Also bi-directional DC-DC converters exist. Such bi-directional DC-DC converters may be used in uninterruptable power supplies (UPS), energy storage applications for renewable energy sources such as solar power, wind power—
US 2011/0317452 describes a bi-directional DC-DC resonant converter with lossless (soft) switching with controlled voltage level in both directions. This is achieved by the addition of an inductor Lnew in the different converter circuits in this publication. The disadvantage of these circuits is that the additional inductor Lnew will increase the power losses and hence reduce the power efficiency.
WO 2013/095161 describes a LLC bidirectional resonant converter comprising: a resonant tank, a first switching circuit connected to the resonant tank via first power conduits, a second switching circuit connected to the resonant tank via second power conduits, a switching element, and at least one switchable inductive element which is arranged by the switching element to be in parallel across the second power conduits when operating in a first mode of operation and arranged by the switching element to be in parallel across the first power conduits when operating in a second mode of operation.
FIG. 1a of the present application shows FIG. 5 of WO 2013/095161. Here, the switching element 517 is switching the resonant circuit between the states shown in FIG. 1b and FIG 1c respectively. In the first state, when power is transferred from left side to right side, the resonant circuit is a commonly known LLC resonant tank, and in the second state, when power is transferred from right side to left side, the resonant circuit is also a commonly known LLC resonant tank. Here, assuming that the transformer ratio is 1:1, the gain (i.e. the relation between the primary side voltage and the secondary side voltage) will have the same gain for all frequencies for both FIG. 1a and FIG. 1c. This could be a disadvantage in applications with battery back-up, where the battery voltage might be different depending on charging or discharging of the battery.
One object of the invention is to provide a bi-directional DC-DC resonant converter which in particular is suitable for charging and discharging batteries in renewable power system applications.
The object of the invention is to provide an improved bi-directional DC-DC resonant converter. As for all types of converters, it is desired that the power efficiency is high (i.e. low losses), and that the costs are low.